Critical dimension (CD) uniformity of photoresist island patterns using alternating phase shifting mask

ABSTRACT

A photoresist film is patterned into an array of island shapes with improved critical dimension uniformity and no phase edges by using two alternating phase shifting masks (AltPSMs) and one post expose bake (PEB). The photoresist layer is exposed with a first AltPSM having a line/space (L/S) pattern where light through alternating clear regions on each side of an opaque line is 180° phase shifted. Thereafter, there is a second exposure with a second AltPSM having a L/S pattern where opaque lines are aligned orthogonal to the lengthwise dimension of opaque lines in the first exposure, and with alternating 0° and 180° clear regions. Then, a PEB and subsequent development process are used to form an array of island shapes. The double exposure method enables smaller island shapes than conventional photolithography and uses relatively simple AltPSM designs that are easier to implement in production than other optical enhancement techniques.

RELATED PATENT APPLICATION

This application is related to the following: U.S. Pat. No. 8,865,008; which is assigned to a common assignee and is herein incorporated by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to a method of improving CD uniformity in photoresist patterns comprised of island arrays, and in particular to a sequence of exposing a photoresist film with a first alternating phase shift mask (AltPSM) having parallel opaque horizontal lines separated by alternating clear 0° and 180° phase regions, and then exposing with a second AltPSM having parallel opaque vertical lines with clear alternating 0° and 180° phase regions, and finally performing a post-expose bake and developing the photoresist film to form substantially circular or elliptical island shapes that are subsequently transferred by etch processes through a stack of underlying layers thereby forming devices such as magnetic tunnel junctions (MTJs) for semiconductor or hard disk drive (HDD) applications.

BACKGROUND

The fabrication of memory devices in semiconductor and HDD applications requires the use photolithography processes to define a pattern in a substrate. A photoresist layer on the substrate is patternwise exposed with radiation that passes through a mask (reticle) having opaque and transparent regions. After the exposed photoresist layer is developed to form a pattern therein, the pattern is etch transferred into the substrate. A common mask is a binary mask comprised of an opaque material such as chrome on a transparent substrate that is typically quartz. Light from a source such as a 248 nm or 193 nm excimer laser passes through regions not blocked by chrome on the mask and presents an aerial image on the photoresist layer. An optical reduction system comprised of a plurality of lenses is used to reduce the size of the mask pattern by a factor of 5:1, for example, in the aerial image impinging upon the photoresist surface. The aerial image has high intensity corresponding to light passing through transparent mask regions, and low intensity (dark areas) where light has been blocked by opaque regions thereby producing “exposed” and “unexposed” regions in the photoresist. A small amount of radiation does reach “unexposed” regions, particularly at borders with “exposed” regions because of diffracted light. This condition limits the minimum feature size that is formed or resolved in the photoresist film. Since there is a constant demand to decrease feature size in order to build a higher density of devices per unit area, numerous resolution enhancement techniques have been developed.

The minimum feature size that can be printed in a photoresist film is defined as R=kλ/NA where R is the minimum resolution, k is a constant for the photolithography process, λ is the exposing wavelength, and NA is the numerical aperture of the projection optics in the exposure tool. A combination of lower k through improved process control, lower λ, and higher NA has enabled a steady reduction in technology nodes during the past 20 years from 180 nm to 45 nm and below. Note that k may also be reduced by enhancements in the mask or photolithography process including the use of attenuated masks, off-axis illumination (OAI), optical proximity correction (OPC), and other means to improve contrast between light and dark areas in the aerial image. FIG. 1 depicts a conventional binary mask for printing an array of island shapes where chrome regions 60 are separated by clear quartz regions 61. However, process latitude and resolution are limited because of poor contrast in the aerial image, especially for photoresist feature sizes that are <250 nm. The pitch p is defined as the sum of linewidth s and the adjacent space v.

A commercial photoresist solution is typically comprised of a polymer, casting solvent, and multiple additives including a photosensitive material that is coated on a wafer and baked on a hot plate to remove substantially all of the solvent and leave a photoresist film. The photosensitive component reacts when exposed to light, and generates an acid, which causes either a deprotection mechanism in a positive tone photoresist where the polymer becomes soluble in a developer solution while unexposed regions remain insoluble, or initiates a crosslinking mechanism in a negative tone photoresist where polymers crosslink to render the exposed regions insoluble in developer while unexposed regions are soluble.

The basic idea behind alternating phase shift masks (AltPSM) is to modify the binary reticle so that alternating clear regions will cause light to be phase shifted 180°. Since the intensity of regular phase light and the intensity of reversed phase light cancel each other in the nominally dark areas, image contrast is improved compared with non-phase shifted masks and attenuated phase shifted masks. However, there are a number of problems with AltPSM. When the layout consists of island type features used to fabricate MTJs, light from adjacent areas that is 180° out of phase will interfere destructively and result in phase edges that are unwanted connections between island features in a positive tone photoresist. As shown in FIG. 2, phase edges 71 generally appear as linear sections between the island shapes 70 in a photoresist patterned on a substrate 8 with a conventional AltPSM. Here, the phase edges are aligned vertically (y-axis direction), but may also appear as horizontal linear sections depending on mask design.

To avoid the phase edge issue, one approach is to use a negative tone photoresist. However, negative tone photoresists have drawbacks including swelling during develop, poor step coverage, a toxic stripper requirement, and sensitivity to ambient oxygen. Another possible corrective measure is introducing grated phase shifted features such as 0° to 60°, 60° to 120°, and 120° to 180° in the mask. This approach requires additional steps in reticle fabrication and a complicated mask design, and is not applicable for very tight (small) pitches. Alternatively, a second mask may be used to expose the photoresist so that the phase edges are removed during the development process. This option requires a complicated layout design that is usually not practical.

Current technology does not enable a solution for printing island features having a small pitch less than about 300 nm, and with acceptable critical dimension uniformity of <3 nm along with a suitable process window (exposure latitude and depth of focus) with AltPSM masks in a manufacturing environment. Therefore, a new photolithography process is needed to enable further advances in realizing high density patterns required for semiconductor and magnetic recording devices that rely on island shapes, which are subsequently etch transferred through an underlying stack of layers to form an array of MTJs, for example.

SUMMARY

One objective of the present disclosure is to provide a method involving AltPSM technology to print island shaped photoresist patterns with CDs<100 nm and with improved CD uniformity for semiconductor and magnetic devices based on MTJs.

A second objective of the present disclosure is to provide a method for achieving the first objective that is based on an AltPSM design that is readily fabricated and suitable for a manufacturing environment.

According to one embodiment, these objectives are achieved by initially depositing a MTJ stack of layers on a substrate formed on a wafer where the MTJ stack comprises a reference layer, a free layer, and a tunnel barrier between the RL and FL. The RL has a fixed magnetization preferably in a perpendicular to plane direction while the FL has a magnetization that is free to rotate between parallel and antiparallel directions with respect to the RL. There may be a seed layer as the bottommost layer and a capping layer as the uppermost layer in the MTJ stack. A bottom antireflective coating (BARC) or a dielectric antireflective coating (DARC) is deposited on the capping layer and serves to improve CD uniformity during a subsequent photoresist patterning step. A photoresist layer that is preferably a positive tone type is coated on the BARC/DARC and is patternwise exposed twice before a post expose bake and pattern development is performed to form an array of island shapes in rows and columns.

A key feature is that the photoresist layer is first patternwise exposed through a first AltPSM having a plurality of opaque and alternating transparent (clear) regions each with a line shape aligned in a vertical direction (i.e. y-axis direction). The alternating clear regions cause the light during the exposure to be phase shifted 180°. The 0° and 180° clear regions on the first AltPSM have a width w1 and w2, respectively, while each of the opaque regions have a width w3. In some embodiments, w1=w2. Moreover, w3 is variable depending on the desired pitch, which is (w1+w3) or (w2+w3). Typically, the alternating clear regions are 180° out of phase by etching trenches between the chrome lines on a front side of the mask. Thus, a first set of trenches corresponding to 0° clear regions has a first depth t1 into the quartz mask while a second set of trenches corresponding to 180° clear regions has a second depth t2 where t1 is unequal to t2. The first patternwise exposure is performed while the wafer is on a wafer stage in an exposure tool that may have a 193 nm or 248 nm excimer laser light source, or an extreme UV light source with a 13.5 nm wavelength.

Thereafter, the wafer remains on the wafer stage and is subjected to a second patternwise exposure through a second AltPSM having a plurality of opaque and alternating transparent (clear) regions each with a line shape aligned in a horizontal direction (i.e. x-axis direction) that is orthogonal to the vertical direction. Again, the alternating clear regions cause the light during the exposure to be phase shifted 180°. The 0° and 180° clear regions on the second AltPSM have a width w4 and w5, respectively, while each of the opaque regions have a width w6. In some embodiments, w4=w5. If circular island shapes are desired, then w1=w4. However, if elliptical island shapes are to be printed, then w1 is greater than or less than w4 depending on the desired long axis direction in the resulting elliptical shapes. A third set of trenches corresponding to 0° clear regions in the second AltPSM has a third depth t3 into the quartz mask while a fourth set of trenches corresponding to 180° clear regions has a fourth depth t4 where t3 is unequal to t4.

In the following step, the wafer is post expose baked on a hot plate to drive the chemical reaction to completion that is caused by the release of photogenerated acid in exposed regions of the photoresist layer. Next, the wafer is developed with an aqueous base solution such as 0.26 N tetramethylammonium hydroxide (TMAH), for example, to wash away exposed regions, and is then rinsed with DI water and dried to provide a pattern of island shapes on the BARC/DARC top surface.

The pattern of island shapes in the photoresist mask layer is then transferred successively through the BARC/DARC and the MTJ stack by a plurality of ion beam etch (IBE) or reactive ion etch (RIE) processes to generate a plurality of islands (MTJ cells) each having a sidewall that extends from a capping layer top surface to the substrate. The photoresist and BARC/DARC are removed during the IBE or RIE processes. In some embodiments, one or more RIE steps may be used with one or more IBE steps for the etch transfer of the island shape through the BARC/DARC and through the MTJ stack. In alternative embodiments, the MTJ stack is replaced by another stack of layers that is patterned into active devices with island shapes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top-down view of a conventional mask design that is used to print a plurality of island features in a photoresist layer.

FIG. 2 is a top-down view of an island array that has been printed in a photoresist layer using a single alternating phase shift mask (AltPSM) according to a conventional method with the unwanted result of creating phase edges between island shapes.

FIG. 3 is cross-sectional view and FIG. 4 is a top-down view of a first AltPSM according to an embodiment of the present disclosure where alternating clear regions on the mask cause light to be phase shifted by 180° and are linear shapes (trenches) having widths w1 and w2 formed on each side of an opaque line.

FIG. 5 is cross-sectional view and FIG. 6 is a top-down view of a second AltPSM of the present disclosure where alternating 0° and 180° clear regions on the mask are linear shapes oriented orthogonal to the linear shapes in the first AltPSM, and have widths w3 and w4 formed on each side of an opaque line.

FIG. 7 is a top-down view of an island array printed in a photoresist layer as a result of first and second patternwise exposures through the first and second AltPSM, respectively, and then performing a post expose bake and development according to an embodiment of the present disclosure.

FIG. 8 is a cross-sectional view of one of the photoresist islands in FIG. 7 that is formed on a BARC or DARC top surface, and above a MTJ stack of layers.

FIG. 9 is a cross-sectional view of the intermediate structure in FIG. 8 after an etch step is used to transfer the island shape through the BARC/DARC.

FIG. 10 is a cross-sectional view after an etch process comprised of a plurality of IBE and/or RIE steps is performed to transfer the island pattern in FIG. 9 through the MTJ stack of layers to form a MTJ cell with a sidewall.

FIG. 11 is a cross-sectional view of the MTJ cell in FIG. 10 after an encapsulation layer is deposited to electrically isolate the MTJ cell from adjacent MTJ cells in the island array.

DETAILED DESCRIPTION

The present disclosure is a method of printing an array of island shapes in a positive tone photoresist layer on a substrate with a process involving two AltPSM to provide island shapes with no phase edges, and improved CD compared with prior art photolithography methods involving AltPSM. The resulting photoresist pattern may be formed on a MTJ stack of layers, or on other stacks of layers that are subsequently patterned into devices for semiconductor or HDD applications. The photoresist pattern may be etch transferred through a MTJ stack to produce an array of MTJ cells used in magnetic random access memory (MRAM) or in spin transfer MRAM (STT-MRAM) applications. In the drawings, each layer is laid out in the (x, y) plane, and a thickness of each layer is in the z-axis direction.

According to a first embodiment of the present disclosure depicted in FIG. 3, a photoresist layer 40 is coated on a BARC or DARC 39 that in turn is formed on substrate 50. The substrate is disposed on a wafer made of Si or the like that typically has a diameter of 6″, 8″, or 12″. A photoresist solution may be dispensed onto the BARC or DARC/substrate stack as the wafer is held on a wafer chuck and rotated in a spin bowl. Then, the wafer is moved to another station that may be a hot plate where a so-called post-apply bake (PAB) is performed to remove substantially all of the casting solvent from the photoresist layer. Next, the wafer is moved to an exposure tool and held on a workpiece (not shown) such as a wafer stage.

Once a well known alignment process of the first AltPSM to the wafer is completed, the photoresist layer is exposed through the first AltPSM 1 a with light 2 from an excimer laser including but not limited to a 193 nm or 248 nm excimer light source in the exposure tool, or with a wavelength proximate to 13.5 nm from an extreme ultraviolent (EUV) source, and impinges on first AltPSM backside 10 b 1. In some embodiments, the light exposure may occur in an immersion lithography system as appreciated by those skilled in the art. Moreover, a topcoat (not shown) may be formed on a top surface of the photoresist prior to the first exposure as a protective measure to prevent water or airborne chemicals from entering the photoresist between exposure and image development. As a result, the deprotection mechanism caused by the photogenerated acid is not disrupted and is able to proceed to completion during a post expose bake. The topcoat is water soluble and is removed during development of the photoresist pattern.

The first AltPSM is made of a quartz substrate 10, and opaque features 11 a each having a width w3 and thickness d that are formed on a side 10 s of the first AltPSM facing the photoresist film 40. Only a portion of AltPSM 1 a is shown in order to simplify the drawing. It should be understood that an AltPSM may have millions of opaque features (i.e. lines) that are made of chrome or the like.

There are clear regions in the first AltPSM that are 180° phase shifted on opposite sides of each opaque feature 11 a. One of the clear regions is comprised of trench 20 having width w1 and backside 20 b, and a portion 20 c of quartz substrate 10 of width w1 between trench backside 20 b and mask backside 10 b 1. A second clear region is comprised of trench 21 having width w2 and backside 21 b, and a portion 21 c of the quartz substrate of width w2 between trench backside 21 b and mask backside 10 b 1. According to one embodiment w1=w2. In one embodiment, the first clear region is a 0° clear region where trench 20 has a depth t1, and the second clear region is a 180° clear region where trench 21 has a depth t2 where t2>t1. However, in alternative embodiments, the first clear region may be a 180° clear region while the second clear region is a 0° clear region. Trenches are formed during mask fabrication by etching into AltPSM 1 a using chrome features 11 a as an etch mask. Dimensions w1, w2, and w3 are aligned in a y-axis direction during the first exposure.

Referring to FIG. 4, a top-down view of first AltPSM 1 a is illustrated. Again, the number of line shapes is significantly reduced from an actual mask in order to simplify the drawing. In the exemplary embodiment, the opaque regions 11 a are line shapes, and the alternating 0° and 180° clear regions comprised of trenches 20, 21, respectively, are also line shapes that have a lengthwise dimension along the x-axis direction. Although width w3 of the opaque regions is shown to be larger than w1 and w2 of the clear regions, w3 may be equal to or smaller than w1 and w2 in alternative embodiments. The chrome lines and clear regions (spaces) form a repeating line/space (L/S) pattern with a pitch that is (w1+w3) or (w2+w3). For example, in one embodiment where w1=w2=w3, the lines and spaces form a 1:1 L/S pattern. It should be understood that the light (not shown) in the aerial image impinging on the photoresist layer 40 in FIG. 3 has a 1:1 L/S pattern but the L/S pattern is reduced by an optical reduction factor N that is an integer greater than 1, such as 5 or 10, for example. Thus, with a 5:1 optical reduction system where N=5, the width of the unexposed regions in the photoresist layer will be ⅕×w3 and the width of exposed regions will be ⅕×w1 (and ⅕ w2).

Referring to FIG. 5, during the second exposure step of the present disclosure, the substrate 50 and wafer (not shown) continue to be held on the same wafer stage in the same exposure tool as used in FIG. 3. In this case, the first AltPSM is removed and a second AltPSM 1 b is aligned above the previously exposed photoresist layer from FIG. 3. During the second exposure, chrome lines 11 b on the second AltPSM are aligned orthogonal to the position of chrome lines 11 a in the first exposure. Thereafter, a second exposure with light 2 is performed.

The second AltPSM 1 b is made of a quartz substrate 10, and opaque features 11 b each having a width w6 and thickness d that are formed on a side 10 s of the second AltPSM facing the photoresist film 40. There are clear regions in the second AltPSM that are 180° phase shifted on opposite sides of each opaque feature 11 b. One of the clear regions is comprised of trench 30 having width w4 and backside 31 b, and a portion 30 c of quartz substrate 10 of width w4 between trench backside 30 b and mask backside 10 b 2. A second clear region is comprised of trench 31 having width w5 and backside 31 b, and a portion 31 c of the quartz substrate of width w5 between trench backside 31 b and mask backside 10 b 2. According to one embodiment w4=w5. In one embodiment, the first clear region is a 0° clear region where trench 30 has a depth t3, and the second clear region is a 180° clear region where trench 31 has a depth t4 where t4>t3. However, in alternative embodiments, the first clear region may be a 180° clear region while the second clear region is a 0° clear region. The trenches are formed by etching into AltPSM 1 b using chrome features 11 b as an etch mask during mask fabrication. Dimensions w4, w5, and w6 are aligned in a x-axis direction during the second exposure.

FIG. 6 depicts a view of a portion of the second AltPSM 1 b from a perspective of the photoresist surface 40 t during the second exposure. In the exemplary embodiment, the opaque regions 11 b are line shapes, and the alternating 0° and 180° clear regions comprised of trenches 30, 31, respectively, are also line shapes that have a lengthwise dimension along the y-axis direction. Although width w6 of the opaque regions is shown to be larger than w4 and w5 of the clear regions, w6 may be equal to or smaller than w4 and w5 in alternative embodiments. The chrome lines and clear regions (spaces) have a repeating line/space (L/S) pattern with a pitch that is (w4+w6) or (w5+w6). The L/S pattern in the second AltPSM is reduced by an optical reduction factor N that is an integer >1, such as 5 or 10 in the aerial image impinging on the photoresist. Accordingly, with a 5:1 optical reduction system, the width of the unexposed regions in the photoresist layer will be ⅕×w6, and the width of exposed regions will be ⅕×w4 (and ⅕ w5).

After the second exposure, the wafer with substrate 50 and overlying photoresist layer 40 is moved to a hot plate where a post exposure bake (PEB) is performed at a temperature between 90° C. and 150° C. for 1 to 2 minutes although other PEB temperatures and times may be acceptable depending on the photoresist composition. Note that the exposure tool is preferably linked to a wafer track where a wafer is moved from one station to another to complete a particular step. Thus, the photoresist coating, PAB, PEB steps are preferably completed at separate stations that are all connected to the wafer track. In addition, there is at least one developer station linked to the wafer track. The developer station is usually comprised of a wafer chuck inside a spin bowl. Following the PEB step, the wafer is moved to a developer station where an aqueous base solution is dispensed onto the photoresist film while the wafer is rotated. After a certain amount of time has elapsed, a DI water solution is dispensed onto the photoresist while the wafer continues to spin in the spin bowl. Finally, the wafer is spun for a plurality of seconds after the water dispensing is stopped to leave a pattern in the photoresist layer that now has an essentially dry surface.

FIG. 7 is a top-down view of photoresist layer following the image development step at the developer station. Each of the resulting photoresist island shapes 40 i have a width w6′ in the x-axis direction and length w3′ in the y-axis direction where w6′ is proximate to 1/N×w6 in the second AltPSM, and where w3′ is proximate to 1/N×w3 in the first AltPSM. According to one embodiment where w3=w6, essentially circular island shapes 40 i are formed when essentially equivalent exposure doses are used for the first and second patternwise exposures. In other embodiments where w3 is unequal to w6, elliptical shapes are produced wherein the long axis is in the x-axis direction (w6′>w3′), or wherein the long axis is in the y-axis direction (when w3′>w6′). The present disclosure also encompasses an embodiment where the relative doses of the first and second exposure may be another technique used to adjust the island shape. For example, when w3=w6, the first patternwise exposure may comprise a different dose than the second patternwise exposure thereby yielding an elliptical shape after subsequent PEB and development steps. Thus, if the first patternwise exposure employs a higher dose than the second patternwise exposure, the photoresist is effectively “overexposed” in the y-axis direction relative to the x-axis direction which means the resulting photoresist islands will have w3′<w6′. BARC or DARC 39 is exposed in openings between the island shapes after the development step.

Referring to FIG. 8, a cross-sectional view of a photoresist island 40 i and underlying stack of layers is taken at plane 44-44 in FIG. 7. Only one island is shown in order to provide an enlarged view. Each photoresist island 40 i has sidewall 40 s and length w3′ in the y-axis direction. In this embodiment, substrate 50 may be a MTJ stack of layers having a bottommost seed layer 51 formed on a bottom electrode 5, for example. A reference layer 52, tunnel barrier 53, free layer 54, Hk enhancing layer 55, and capping layer (hard mask) 56 are sequentially formed on the seed layer. However, the present disclosure is not limited to the bottom spin valve configuration shown in FIG. 8 and anticipates that other MTJ configurations may be used in MTJ stack 50. The compositions of the MTJ layers are found in related U.S. Pat. No. 8,865,008 and are not described herein. In alternative embodiments, MTJ stack 50 may be replaced with one or more layers that are subsequently patterned using island shape 40 i as an etch mask to form devices in semiconductor or HDD applications.

Referring to FIG. 9, photoresist island shape 40 i in FIGS. 7-8 serves as an etch mask while a first etch is performed that may be an ion beam etch (IBE) or reactive ion etch (RIE) to transfer the shape 40 i through the BARC or DARC 39 thereby yielding BARC/DARC sidewalls 39 s that are coplanar with sidewall 40 s.

Thereafter, in FIG. 10, one or more IBE or RIE steps, or a combination of IBE and RIE steps are performed to transfer the island shape having length w3′ and width w6′ through the MTJ stack 50 and stop on top surface 5 t of the bottom electrode thereby yielding a MTJ cell 50 a. A plurality of other MTJ cells is formed simultaneously with MTJ cell 50 a but only one cell is depicted to simplify the drawing. Typically, photoresist islands 40 i and BARC/DARC 39 are etched away during the MTJ etch to leave a sidewall 80, and a top surface 56 t on the MTJ hard mask 56 that has a length essentially equal to w3′ and a width w6′. Depending on the nature of the MTJ etch process, MTJ sidewall angle α may be from 70° to 90°.

Referring to FIG. 11, an encapsulation layer 75 that may include first sub-layer 75 a with thickness d1, and second sub-layer 75 b with thickness d2 is deposited by a physical vapor deposition (PVD) method or the like to fill openings between MTJ cell 50 a and adjacent MTJ cells that are not shown. According to the exemplary embodiment, the first sub-layer is conformally deposited on bottom electrode top surface 5 t and on MTJ sidewall 80. A planarization method such as a chemical mechanical polish (CMP) process is used to form a MTJ hard mask top surface 56 t that is coplanar with a top surface 75 t of the encapsulation layer. Thereafter, a conventional series of steps are employed to form a top electrode and overlying layers (not shown) on the MTJ cell 50 a and encapsulation layer to complete the fabrication of a MRAM or STT-MRAM device.

The photolithography sequence involving two AltPSM as disclosed herein enables a smaller resolution limit for island features, and improved CD uniformity compared with binary and attenuated phase shift masks. Moreover, the AltPSM mask fabrication is simplified compared with more complicated designs used in existing AltPSM technology to avoid phase edge issues. Furthermore, the L/S designs on the two AltPSM are considerably easier to produce than in current process flows where a first AltPSM is employed to form an island shape, and then a second AltPSM with a complicated design is required to clear the phase edges. Although exposure of a negative tone photoresist through an AltPSM conceptually avoids phase edge concerns, this technique introduces new problems that currently have no acceptable solution. The process sequence of the present disclosure may be readily implemented in existing manufacturing lines since no new tools or materials are required.

While this disclosure has been particularly shown and described with reference to, the preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of this disclosure. 

We claim:
 1. A method of forming an array of island shapes in a photoresist layer, comprising: (a) providing a positive tone photoresist layer on a substrate; (b) performing a first patternwise exposure of the photoresist layer through a first alternating phase shifting mask (AltPSM) comprised of a first set of parallel opaque lines each having a width w3 and aligned in a first direction on a first transparent substrate relative to the photoresist layer, wherein each of the first set of parallel opaque lines has a first clear region having a width w1 on one side and a second clear region having a width w2 on a second side thereof such that light during the first patternwise exposure that passes through the first and second clear regions is phase shifted by 180°, and the first and second clear regions alternate across the first transparent substrate; (c) performing a second patternwise exposure of the photoresist layer through a second AltPSM without moving the substrate after the first patternwise exposure, wherein the second AltPSM is comprised of a second set of parallel opaque lines aligned in a second direction orthogonal to the first direction, and are formed on a second transparent substrate, and wherein each of the second set of parallel opaque lines has a width w6, and has a third clear region having a width w4 on one side and a fourth clear region having a width w5 on a second side thereof such that light passing through the third and fourth clear regions is phase shifted by 180°, and the third and fourth clear regions alternate across the second transparent substrate; (d) performing a post-expose bake (PEB) of the photoresist layer on the substrate after the second patternwise exposure; and (e) applying a developer solution to photoresist layer on the substrate to remove first and second patternwise exposed regions and thereby form an array of island shapes that are unexposed regions of the photoresist layer.
 2. The method of claim 1 wherein the first and second patternwise exposures are accomplished with an exposure tool comprised of a 193 nm or 248 nm excimer laser, or comprised of an extreme ultraviolet (EUV) source having a exposure wavelength proximate to 13.5 nm.
 3. The method of claim 2 wherein the first and second patternwise exposures comprise an immersion lithography technique.
 4. The method of claim 1 wherein the first and second clear regions are comprised of first and second trenches, respectively, in the transparent substrate, and wherein the first trench has a thickness t1 and the second trench has a thickness t2 where t2 is unequal to t1.
 5. The method of claim 1 wherein w3=w6 such that circular island shapes are formed in the photoresist layer, or w3 is unequal to w6 such that elliptical island shapes are formed in the photoresist layer.
 6. The method of claim 1 wherein w3=w6, and a higher exposure dose is employed for one of the first and second patternwise exposure relative to the exposure dose used for the other of the first and second patternwise exposure thereby forming an elliptical island shape after the PEB and application of the developer solution.
 7. The method of claim 1 wherein the substrate has a bottom antireflective coating (BARC) or a dielectric antireflective coating (DARC) as an uppermost layer.
 8. The method of claim 1 further comprised of forming a topcoat on the photoresist layer before performing the first patternwise exposure.
 9. The method of claim 1 wherein w1=w2, and w4=w5.
 10. The method of claim 1 wherein each of the island shapes has a widthwise dimension w3′ in the first direction and a lengthwise dimension w6′ in the second direction where w3′ is 1/N×w3, and w6′ is 1/N×w6, and where N is an optical reduction factor that is an integer greater than
 1. 11. The method of claim 1 wherein the first and second sets of opaque lines are made of chrome, and the first and second transparent substrates are a quartz material.
 12. A method of fabricating an array of magnetic tunnel junction (MTJ) cells on a substrate, comprising: (a) providing a MTJ stack of layers on the substrate, and a bottom antireflective coating (BARC) or a dielectric antireflective coating (DARC) on a top surface of the MTJ stack; (b) coating a positive tone photoresist layer on the BARC or DARC; (c) performing a first patternwise exposure of the photoresist layer through a first alternating phase shifting mask (AltPSM) comprised of a first set of parallel opaque lines each having a width w3 and aligned in a first direction on a first transparent substrate relative to the photoresist layer, wherein each of the first set of parallel opaque lines has a first clear region having a width w1 on one side and a second clear region having a width w2 on a second side such that light passing through the first and second clear regions during the first patternwise exposure is phase shifted by 180°, and the first and second clear regions alternate across the first transparent substrate; (d) performing a second patternwise exposure of the photoresist layer through a second AltPSM without moving the substrate after the first patternwise exposure, wherein the second AltPSM is comprised of a second set of parallel opaque lines aligned in a second direction orthogonal to the first direction, and are formed on a second transparent substrate, and wherein each of the second set of parallel opaque lines has a width w6, and has a third clear region having a width w4 on one side and a fourth clear region having a width w5 on a second side such that light passing through the third and fourth clear regions is phase shifted by 180°, and the third and fourth clear regions alternate across the second transparent substrate; (e) performing a post-expose bake (PEB) of the photoresist layer on the substrate after the second patternwise exposure; (f) applying a developer solution to photoresist layer on the substrate to remove first and second patternwise exposed regions and thereby form an array of island shapes in unexposed regions of the photoresist layer; and (g) transferring the island shapes through the BARC or DARC, and then through the MTJ stack of layers with a plurality of etch steps.
 13. The method of claim 12 wherein the first and second patternwise exposures are accomplished with an exposure tool comprised of a 193 nm or 248 nm excimer laser, or comprised of an extreme ultraviolet (EUV) source having a exposure wavelength proximate to 13.5 nm.
 14. The method of claim 13 wherein the first and second patternwise exposures comprise an immersion lithography technique.
 15. The method of claim 12 wherein the first and second clear regions are comprised of first and second trenches, respectively, in the transparent substrate, and wherein the first trench has a thickness t1 and the second trench has a thickness t2 where t2 is unequal to t1.
 16. The method of claim 12 wherein an exposure dose for the first patternwise exposure is essentially equal to an exposure dose for the second patternwise exposure, and w3=w6 such that circular island shapes are formed in the photoresist layer, or w3 is unequal to w6 such that elliptical island shapes are formed in the photoresist layer.
 17. The method of claim 12 wherein the substrate is a bottom electrode in a magnetic random access memory (MRAM) or spin transfer-MRAM (STT-MRAM) device.
 18. The method of claim 12 further comprised of forming a topcoat on the photoresist layer before performing the first patternwise exposure.
 19. The method of claim 12 wherein w1=w2, and w4=w5.
 20. The method of claim 12 wherein each of the island shapes has a widthwise dimension w3′ in the first direction and a lengthwise dimension w6′ in the second direction where w3′ is 1/N×w3, and w6′ is 1/N×w6, and where N is an optical reduction factor that is an integer greater than
 1. 21. The method of claim 12 wherein the first and second sets of opaque lines are made of chrome, and the first and second transparent substrates are a quartz material. 